This invention relates to improved ultraviolet light curable silicone release coating compositions. More particularly, it relates to blends of pre-crosslinked epoxy functional polydiorganosiloxane silicone polymers fluids which incorporate long chain polydimethylsiloxane blocks in their molecular structure along with lower molecular weight linear epoxyfunctional silicone polymers. Such blends are efficiently crosslinked when irradiated with ultraviolet light in the presence of compatible cationic photocatalysts and are particularly well suited for paper release applications.
Silicone compositions have long been used for rendering surfaces non-adherent to materials which would normally adhere thereto. For a long time, it was necessary that these silicone coatings be applied as a dispersion within a solvent in order to control the viscosity of the coating material so as to be suitable for coating applications. However, although the solvent aids in the application of the coating, it is an inefficient process as the solvent must thereafter be evaporated. The evaporation of solvent requires large expenditures of energy. Additionally, pollution abatement procedures require that solvent vapors be prevented from escaping into the air. Removal and recovery of all of the solvent entails considerable expenditure for apparatus and energy.
It has been thus noted that there is a need to provide a solventless coating composition that will however remain easy to apply to the substrate. Such solventless coating compositions are sometimes referred to as "100% solids" compositions. The absence of solvent in such compositions lowers the amount of energy required to effect a cure and eliminates the need for expensive pollution abatement equipment. The present invention provides an improved solvent-less pre-crosslinked epoxy functional polydiorganosiloxane fluid that will cure to a non-adherent surface when combined with an effective amount of a linear alkylate diaryl iodonium salt and exposed to ultraviolet radiation.
Epoxy functional; silicone release agents are constrained by the need to balance both fast cure and photocatalyst miscibility with a low force or premium release. Good release performance coupled with an efficient photocure response is obtained when the epoxy equivalent weight is held between about 800 and about 1600. If the epoxy group in the epoxy-silicone is derived from 4-vinylcyclohexene oxide (VCHO), then the organofunctional content of the silicone must therefor vary between about 8 and about 16 weight percent. A high epoxy content, one that is above about 16 weight percent in cured epoxy-silicone coatings fails to release properly, while low epoxy contents, below about 8 weight percent in cured epoxy-silicone coatings results in a slow curing coating that suffers from an additional problem in that the iodonium catalyst tends to rapidly settle out of the silicone fluid upon standing.
Release coatings are useful for many applications whenever it is necessary to provide a surface or material that is relatively non-adherent to other materials which would normally adhere thereto. Silicone paper release compositions are widely used as coatings which release pressure-sensitive adhesives for labels, decorative laminates, transfer tapes, etc. Silicone release coatings on paper, polyethylene, Mylar.RTM. and other such substrates are also useful as non-stick surfaces for food handling and industrial packaging applications.
For example, when labels are coated with an adhesive, it is desirable that the paper backing be easily peeled away from the label when it is ready for use, yet the adhesive quality of the label should not be derogated by the fact that it has been peeled away from the substrate upon which it was stored. The same principle applies to certain types of adhesive tapes which come in rolls. It is necessary that the tape unroll easily and still maintain its adhesive characteristics. This can be accomplished by coating the non-adhesive side of the tape with a silicone release composition that will come into contact with the adhesive as the roll of tape is manufactured.
Silicone release compositions are often sold as dispersions of reactive polysiloxanes in organic solvents such as toluene, or as emulsions in water. A cross-linking catalyst, also known as the curing agent, is then added to the polysiloxane-solvent mixture. The coating compositions is applied to a substrate which is passed through an oven to evaporate the carrier fluid and cure the silicone to an non-adherent or "adhesive" surface. As noted above, this process is quite energy intensive since it requires high oven temperatures to evaporate the solvent and effect the cure at commercially useful speeds.
Use of these solvent based products is becoming increasingly unattractive because of rising energy costs and stringent regulation of solvent emissions into the atmosphere. Other solventless silicone release compositions such as that described in U.S. Pat. No. 4,256,870 herein incorporated by reference have addressed the environmental problem of hydrocarbon emission but still require high oven temperatures for proper cure.
Optimum energy savings as well as necessary ecological considerations are both served by radiation curable compositions. Specifically, an ultraviolet (UV) radiation curable 100% solids silicone release system eliminates the need for high oven temperatures and for expensive solvent recovery systems, and is, therefore, a useful and commercially desirable product.
UV curable silicone compositions are not unknown. A patent issued to R. V. Viventi, U.S. Pat. No. 3,816,282, Jun. 11, 1974, and assigned to the General Electric Company, describes a room temperature vulcanizable silicone composition (RTV) in which a mercaptoalkyl substituent attached to polysiloxanes add to vinyl functional siloxanes in a free-radical process upon UV irradiation in the presence of free-radical type photosensitizers. The particular compositions described by Viventi cure too slowly to be useful for paper release applications. Furthermore, the use of mercaptoalkyl photoreactive substituents gives rise to offensive odors both in product manufacture and in cured materials.
Ultraviolet radiation will initiate free-radical cross-linking in the presence of common photosensitizers which are well-known to persons familiar with the art of radiation curing mechanisms. However, silicone compositions that utilize photosensitizers (such as benzophenone) as a curing agent also require stabilizers (such as hydroquinone) to prevent premature reaction and provide reasonable shelf-life.
Commonly available photosensitizers are only slightly soluble in polydimethylsiloxane fluids which are the basic starting materials for silicone coating compositions. Low solubility of these photsensitizers causes problems in selection of the necessary ingredients. A further difficulty inherent in free-radical systems is oxygen inhibition which necessitates that the coated substrates be under an inert atmosphere while undergoing irradiation in order to cure within a reasonable amount of time. Use of an inert atmosphere adds a complication and an expense to the coating and curing process.
It has previously been discovered that UV curable epoxy functional silicones which are suitable for release coating applications fall into a narrow range of epoxy content and viscosity. The limits to these parameters are imposed by the necessity of coating 0.1 to 0.3 mil layers of these silicone fluids onto various substrates, and by the necessity for these formulations to cure quickly upon exposure to UV radiation and while adhering well to the substrate.
The requirement that these epoxy functional silicones be applied in thin coats dictates that the fluids be of low viscosity such as, for example, approximately 100 to 2,000 centistokes. Consequently, the epoxy functional silicones typically must be low molecular weight fluids. Also, the efficiency of the curing catalyst must be high in order to provide sufficient cross-linking and the formation of a tight, smear-resistant coating which adheres well to the substrate.
The present invention represents an improvement over U.S. Pat. No. 4,297,717 to Eckberg et al. wherein it has been discovered that small amounts, varying between 5 and 25 weight percent, of high viscosity pre-crosslinked epoxysilicones that incorporate blocks of long chain linear non-functionalized polydimethylsiloxane in their molecular structure, when dispersed in conventional low viscosity epoxy-functional silicones such as those taught in the Eckberg '717 patent, and photocured in the presence of compatible iodonium photocatalysts, provide a novel cured release coating requiring much lower force to initiate tear away or peel away from aggressive pressure sensitive adhesives than the force that would be required for a photocured low viscosity expoxysilicone coatings absent the high viscosity pre-crosslinked additives.
The requirement for a highly efficient photo initiator severely restricts the structure of the catalyst since it also must be capable of dissolving or dispersing well in the epoxy functional silicone fluid. U.S. Pat. No. 4,310,469 herein incorporated by reference discloses a UV initiated cationic ring opening curable curing mechanism for dimethyl epoxy chain-stopped linear polydimethylsiloxane fluids utilizing bisaryliodonium salts of the following formula, wherein X=SbF.sub.6, PF.sub.6 or BF.sub.4 and wherein R is a C.sub.(4-20) organo radical selected form alkyl and haloalkyl and mixtures thereof and n is a whole number equal to 1 to 5, inclusive. The catalysts described by Crivello are thick, high viscosity liquids or waxy solids which disperse poorly in the low molecular weight epoxy functional silicones utilized by the present invention. These catalysts exhibit the typical solubility characteristics of diaryliodonium salts, namely, being soluble in polar organic solvents such as chloroform and acetone but insoluble in non-polar organic solvents such as pentane, hexane and petroleum ether. Such solubility behavior severely limits the utility of these salts for initiating the rapid photocuring of epoxy functional silicone paper release compositions.
Although Crivello discloses that R may equal organo radicals selected from alkyl, haloalkyl and branched alkyl groups containing from 4 to 20 carbon atoms, the unique characteristics of "linear alkylate" bis(dodecylphenyl) iodonium salts such as are disclosed by the present invention are not recognized by Crivello. These bis(dodecylphenyl) iodonium salts will rapidly dissolve in the polysiloxane base polymer fluid and disperse throughout, thereby being an efficient photo initiator agent. Such salts are particularly well adapted for use with the novel epoxy functional silicone coating compositions herein provided.
Epoxy functional silicone paper release coating compositions must ordinarily have epoxy contents of less than approximately 16 weight percent because of the end uses to which such coatings will be put, namely, to serve as non-adherent surfaces capable of releasing aggressive pressure sensitive adhesives. When the epoxy content of the silicone compositions greater than about 16 weight percent, excessive force is required to remove adhesive coated articles from the cured silicone coatings. Note, however, that this may be a useful characteristic whenever it is desirable to selectively control the release characteristics of an adhesive.
It is therefore an object of the instant invention to provide low or premium release silicone coatings that are efficiently photocurable. It is another object of the present invention to provide ultraviolet light curable epoxy functional silicone coating compositions.
It is another object of the instant invention to provide ultraviolet light curable epoxy-functional silicone coating compositions.
It is a further object of the present invention to provide a process for the preparation of epoxy-functional silicone coating compositions incorporating long linear chain non-functional polydimethylsiloxane blocks in their molecular structure.
It is a further object of the instant invention to provide a method for rendering surfaces non-adherent to materials that would normally adhere thereto.
It is a further object of the instant invention to provide products possessing non-adherent surfaces comprising an ultraviolet light cured epoxy functional silicone coating.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of the invention.